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slight vibration at idle

california1999

NAXJA Forum User
Location
california
Ok so i just replaced the tps on my xj and cleaned the idle air control valve, and still get a slight vibration when my jeep is at idle. I have noticed that the rpm's do spike up when it does this. What could this be? Should i just replace the idle air control valve all together? i have a 99 xj 4.0 with about 160xxx miles
 
So, what makes you feel that a vibration would be related to the tps or iac?

The iac is a stepper motor that extends/retracts on command from the PCM, so the PCM is attempting to adjust the engine speed in response to some data feed back it is receiving.

I would start with checking the condition of your plugs, cleaning the throttle body, etc. to see if that improves the idle and eliminates the vibration.
 
New plugs can foul in a few hundred miles if there is a fuel, air, oil leak, valve problem or cracked head.....
 
Pull all the plugs, keeping them in order of the cylinders they came from, then check the condition of each plug and compare them to each other.

Post up what you find--a photo would be great.
 
Maybe your Vibration Dampener is starting to come apart.

A common part failure at your mileage.
 
We'll I did have a craked header and drove it like that for about 8 months, so thear might be a chance I fould the plugs. I also thought about motor mounts but I don't think that's it because my rpm spikes up when I am at idle and that's when I feel the vibration.
 
if it's more like a miss, have you ruled out vacuum leaks? I know mine ran quite a bit smoother once I fixed a big vacuum leak on the passenger side fender.

Just this weekend, whilst troubleshooting my fuel delivery problem, I found several vacuum elbows that were cracked and leaking at the intake manifold and the fuel regulator. Hadn't seen them before.
 
ok so i pulled the plugs and thet all looked to be in ok shape. I dont really know what too look for. It does feel like a miss but havent got an engine light. could it be a vacume??
 
Vibration is usually a miss fire. Can be a bad spark or fuel injector issue. Looking at each spark plug can tell you if one cylinder has an issue.

Cap, rotor, plug wires, plugs, fuel injectors, wire contacts to the fuel injectors, bad ground, a vacuum leak in just the right spot...and a few others can cause a miss, vibration at idle. Fastest way to get started is to examine all the plugs.

Try some fuel injector cleaner in the gas tank. Also spray some carburetor cleaner into the throttle bottle while the engine is running. Is the rotor and cap old and dirty on the inside, contact points eroded and oxidized or burned?
Air filter?
 
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